Messenger RNA selection by E. coli ribosomes following infection with bacteriophage T4 may involve interaction with ribosomal protein S1. Our recent discovery that S1 binds fMet-tRNA will be further investigated to determine whether this interaction occurs on the ribosome during the course of protein synthesis initiation. These studies will involve the analysis of f2RNA ribosome binding sites in the presence and absence of both S1 and fMet-tRNA making use of S1-depleted ribosomes and initiation factors as well as B. stearothermophilus ribosomes, which normally lack S1 but which can respond to S1. Chemical and photochemical crosslinking of S1 and fMet-tRNA will be tested using initiation competent ribosomes, labeled fMet-tRNA and antisera to S1. Efforts will also be made to find a temperature sensitive mutant of S1 using localized mutageneses of P1 or T4 transducing particles. A low molecular weight translation inhibitor, discovered earlier by A. Ulhmann and V. Clark, will be purified and assayed in control and T4 infected cells. This factor inhibits binding of fMet-tRNA to ribosomes (Clark), expression of beta-galactosidase in whole cell (Ulhmann) and the synthesis of beta-galactosidase alpha-peptide in vitro from lac mRNA (Lagrimini and Conway).